principles of network and system administration phần 2 pot

24 NETWORK SERVICES AND CONTRACTSrelated to contracts for network services, focusing on issues of quality of service andperformance.2.1 A classification of network services At its most b

20 PRICING AND COMMUNICATIONS NETWORKSThe fact that there exists a Np to make this possible follows from the fact that N p is the Lagrangian multiplier with which we can solve the constrained optimization problem P

(see Appendix A)

Let xi Np/ be the maximizing value of xi in (1.1), expressed as a function of the priceN

p We call x i p/ user i’s demand function It is the amount of bandwidth he would wish

to purchase if the price per unit bandwidth were p Under our assumptions on u i, xi p/ decreases as p increases Let us suppose that at a price of 0 the customers would in aggregate wish to purchase more than C, and when p is sufficiently large they would wish

to purchase less that C It follows that, as p increases from 0, the total amount of bandwidth

that the customers wish to purchase, namelyP

i x i p/, decreases from a value exceeding

C towards 0, and at some value, say p D N p, we haveP

i x i Np/ D C By setting the price at

N

p the operator ensures that the total bandwidth purchased exactly exhausts the supply and

that it is allocated amongst users in a way that maximizes the total benefit to the society

of customers taken as a whole

This solution to problem P has a number of desirable properties First, the network

need not know the utility functions of the users Secondly, the decisions are taken

in a decentralized way, each user rationally choosing the best possible amount ofbandwidth to buy Thirdly, since the price is chosen so that demand equals capacity, thenetwork technology’s sharing policy does not intervene Users decide the sharing amongstthemselves, with price serving as a catalyst Hence, price works as a kind of flow controlmechanism to shape the demand

The operator may or may not be happy with this solution He has obtained a total revenueequal to NpC, which of course equals N pP

i x i Np/ Customer i is left with a ‘user surplus’

of ui xi Np//
Npxi Np/ The total value to society of the Athens–London link has been

maximized and then divided amongst the operator and customers However, it is has notbeen equally divided amongst the customers, nor in a way that specially favours the operator

or takes account of his costs

If our operator is not subject to competition or regulation he might like to capture allthe benefit for himself; he can do this if he can present each customer with a customized

offer He simply says to customer i , ‘you may buy xi Np/ units of bandwidth for a penny less than ui xi Np// — take it or leave it’ User i is better off by a penny if he accepts this offer, so he will do so, but the operator gains all the value of the link, minus N pennies If

the operator cannot make each customer such a take-it-or-leave-it offer, he still might say,

‘you may can have any amount of bandwidth you like, but at a price of pi per unit’ That

is, he quotes different prices to different customers As we see in Section 6.2.1 the operatormaximizes his revenue by quoting higher prices to customers who are less price sensitive

In practice, the operator does not usually know much about his customers, and it is veryunlikely that he knows their utility functions Moreover, he cannot usually tailor prices

to individual customers Nonetheless, we will find that some charging schemes are betterthan others Some schemes give customers a greater incentive to act in ways that maximizewelfare Other schemes enable the operator to obtain a greater payment, thereby obtaining

a greater part of the link’s value for himself

Let us take the second of these first There are various ways in which the operator canextract a greater payment He may present users with nonlinear prices For example, he canmake a subscription charge, or vary the price per unit bandwidth according to the quantity

a user purchases He may offer different prices to different groups of customers (e.g homeand business customers) Or he may define versions of the transport service, such as dayand night service, and offer these at different prices

A GUIDE TO SUBSEQUENT CHAPTERS 21

If the operator is constrained to sell the bandwidth at a single price his objective function

is pP

i x i p/, which may be maximized for a p for which not all of the bandwidth is sold.

Of course he must always have an eye on the competition, on his desire to grow hiscustomer base, and to fund the costs of building, maintaining and expanding his network.Thus far, we have taken a very simple view of both the service and the network Manymodern services are not best provided for by simply allocating them a constant bit rate pipe

A customer’s service requirement is better-visualized as his need to transport a stream ofpackets, whose rate fluctuates over time The customer may be able to tolerate loss of aproportion of the packets, or some delay in their delivery; he may be able to assist thenetwork by guaranteeing that the rate at which he sends packets never exceeds a specifiedmaximum

Suppose that a customer has utility for a transport service that can be characterized interms of some set of parameters, such as acceptable mean packet delay, acceptable peakrate, mean rate, and so on Chapters 2 and 3 describe ways that such services can be

provided Suppose there are J such services types and we label them 1; : : : ; J As we

show in Chapter 4, it can be a good approximation to suppose that the supplier’s link

can simultaneously carry n1; : : : ; nJ connections of each of these services, at guaranteedqualities of bit transfer, provided P

j n jÞj  C, where Þ1; : : : ; ÞJ and C are numbers

that depend on the burstiness of the sources, the link’s resources and the extent to whichstatistical multiplexing takes place The supplier’s problem is to decide how to charge for

these J different services Note that problem P has a new dimension, since the constraint

now involves fÞ1; : : : ; ÞJg

We continue discussion of this problem in Chapter 8 One must be cautious in applyingeconomic models Pricing is an art No single theory can weigh up all the important factorsthat might affect pricing decisions in practice No single prescription can suffice in allcircumstances There are many technology aspects that must be taken into account, such asquality of service, multi-dimensional contracts, network mechanisms for conveying priceinformation, the capabilities to support dynamic prices, and the power and responsibility

of edge devices It is particularly difficult to price a good for which customers havepreferences over attributes that are difficult to measure, such as brand name, servicereliability, accessibility, customer care, and type of billing Marketing strategies that takeaccount of such attributes can lead to prices that seem rather ad hoc This is particularlytrue in the market for communications services

1.5 A guide to subsequent chapters

In Chapters 2–4 of Part A, we expound the fundamental framework and concepts that weuse to think about network services We explain the important concepts of service contractand network control As examples, we describe the services provided by ATM and theInternet We introduce the idea of effective bandwidths, which are the key to addressingquestions of pricing services that have quality of service guarantees

In Chapters 5 and 6 of Part B, we present some key economic concepts that are relevant

to pricing The material in these chapters will be familiar to readers with a background ineconomics and a useful tutorial for others

Part C is on various approaches to pricing and charging for service contracts No oneapproach can be applied automatically in all circumstances The designer of a chargingscheme needs to consider the type of service contract that is being priced, and whetherthe aim of pricing is fairness, cost recovery, congestion control or economic efficiency

22 PRICING AND COMMUNICATIONS NETWORKSChapter 7 describes cost-based pricing methods and discusses how such methods are used

in practice in the telecommunications industry Chapter 8 is concerned with charging forguaranteed contracts (those with certain agreed contract parameters, such as the packetloss probability) Chapter 9 discusses congestion pricing Chapter 10 is concerned withcharging for flexible contracts (those in which certain contract parameters, such as peakrate, are allowed to change during the life of the contract)

Part D concludes with discussions of the special topics of multicasting, interconnection,regulation and auctions (Chapters 11–14) Auctions are of interest because they are oftenused to sell important resources to the telecoms industry Also, auction mechanisms havebeen proposed for allocating network resources to users in real time

1.6 Further reading

There are many excellent books on the digital economy and on the impact of the newtechnologies, especially the Internet Shapiro and Varian (1998) give an economist’sperspective on the rules that govern markets for information goods Kelly (1999) gives

a wonderful introduction to the Internet economy and the new concepts that apply to it.Another well-written book is that of Downes, Mui and Negroponte (2000), which explainsthe interaction between the laws of Metcalf and Moore These laws are, respectively, that

‘the value of a network increases as about the square of the number of users’, and ‘thenumber of transistors in computer chips doubles every eighteen months’ The web pages

of Economides (2002) and Varian (2002) contain references to many papers on issues ofnetwork economics, and pointers to other relevant sites

A great source of articles on the evolution of the Internet and related economic issues isthe home page of Odlyzko (2002), and a good source for information on many issues ofthe Internet telecoms industry is The Cook Report on Internet, Cook (2002) Isenberg and

Weinberger (2001) describe the paradox of the best network : namely, ‘the best network is

the hardest one to make money running’

Network Services and Contracts

It is useful to distinguish between ‘higher-level’ and ‘lower-level’ services Higher-levelservices are those that interface directly with customers Lower-level services are thosethat customers use indirectly and which are invisible to them Consider, for example, theInternet as it is used by students and staff of a university One higher-level service isemail; another is web browsing Web browsing uses the lower-level service of Internetdata transport to exchange data between users’ terminals and the servers where web pagesreside The quality of the higher-level web browsing service depends on the quality of thelower-level transport service That is, the speed at which web pages will be delivered tousers partly depends on the quality of the network’s data transport service This will bespecified in a contract between the university and the network

A transport service can be defined in many ways It can be defined in terms of a guarantee

to transport some amount of information, but without any guarantee about how long thiswill take It can fully specify the performance that is to be provided, and do this at thestart of the service Alternatively, it can respond to changing network load conditions, andcontinuously renegotiate some qualities of the information transfer with the data source

We investigate these possibilities in this chapter

Finally, we note that the provision of a service involves not only a flow of information,but also a flow of value Flow of information concerns data transport, whereas flow ofvalue concerns the benefit that is obtained One or both parties can benefit from the flow

of value However, if one party enjoys most of the value it is reasonable that he shouldpay for the service For example, if an information server sends a customer advertisementsthen the information flow is from server to customer, but the value flow is from customer

to server, since it is the advertiser who profits This suggests that the server should pay If,instead, the customer requests data from the server, then value flows to the customer and

so the customer should pay Note that it is neither the initiator of the transport service, northe one who sends information that should necessarily pay

This chapter is about various characteristics of services, independently of chargingissues In Section 2.1 we discuss a classification of the network services according

to different characteristics We also provide a primer to present technology, in which

we explain the characteristics of the most common network service technologies.Please note that the figures that we quote for various parameters, such as SONET’smaximum line speed of 10 Gbps, are continually changing The concepts that we present

do not depend on such parameter values In Section 2.2, we discuss generic issues

Pricing Communication Networks: Economics, Technology and Modelling.

Costas Courcoubetis and Richard Weber Copyright  2003 John Wiley & Sons, Ltd.

ISBN: 0-470-85130-9

24 NETWORK SERVICES AND CONTRACTSrelated to contracts for network services, focusing on issues of quality of service andperformance.

2.1 A classification of network services

At its most basic level a network provides services for transporting data between points inthe network The transport service may carry data between just two points, in which case

we have a unicast service Or it may carry data from one point to many points, in which case we have a multicast service.

The points between which data is carried can be inside the network or at its periphery.When a web server connects with a user’s browser then both points are at the periphery.When an access service connects a customer’s terminal equipment to the network of

a different service provider then the customer’s point is at the periphery and thepoint connecting to the different service provider’s network is inside When a networkinterconnects with two other networks then both points are inside Thus network operatorscan buy or sell transport services amongst themselves and collaborate to provide transportservices to end-points residing on different networks We see all these things in the Internet.For simplicity, we often refer to a large collection of cooperating networks that provide agiven transport service as ‘the network’

2.1.1 Layering

Service layering is common in communication networks A higher layer service consumeslower layer services and adds functionality that is not available at the lower layers Services

of various layers can be sold independently, and by different service providers An example

of a higher layer service is an end-to-end transport service that connects customer equipment

at two periphery points of the network This service uses lower layer services, some of whichare strictly internal to the network; these lower layer services provide connectivity between

internal nodes of the network and the access service that connects the users’ equipment to

the network The end-to-end service may perhaps add the functionality of retransmittinginformation lost by the lower-level services

A simple analogy can be made by considering a network of three conveyor belts Oneconnects node A to node B Two others connect node B to nodes C and D Suppose thateach conveyor belt is slotted and provided with fixed size bins that move with the belt.Parcels are inserted into the bins so that they do not fall off the belts while travelling Inorder to provide an end-to-end service from A to C and D, some additional functionality

is needed For instance, bins travelling between A and B might be coloured red and blue.Parcels arriving in a red bin at node B are assigned by a clerk to continue their journey onthe conveyor belt from B to C, whereas parcels in the blue bins continue on the conveyorbelt from B to D Clerks are needed to read the destination addresses, fill the differentcolour bins on the conveyor belt, and empty the bins that arrive at nodes C and D Ofcourse there are other ways to build the same end-to-end service, for instance, we coulduse bins of just one colour on the belt from A to B, but have a clerk at node B checkthe destination address of each arriving parcel to decide whether it should next be placed

on belt BC or BD A key feature of this setup is the layering of services: one or morecompanies may provide the basic conveyor services of conveyor belts AB, BC and BD,while another company provides and manages the bins on top of the conveyor belts Yetanother company may provide the service of filling and emptying the bins (especially

A CLASSIFICATION OF NETWORK SERVICES 25

if bins are a single colour and the destination address of the parcels must be checked

at point B) Thus, our setup has three layers of service The first layer is the conveyorservice AB In Internet terms it is analogous to an access service, which connects theequipment of customer A to the network B by, say, a dial-up connection Typically, anInternet service provider provides the other two layers of service (of running the conveyorsinternal to the network, and managing and filling the bins on the conveyors, includingthe access part) Sometimes, a third party provides all three layers of service in theaccess part

Let us illustrate these concepts in more depth by briefly describing transport servicelayering in an actual example from the current Internet We view the Internet as a singlenetwork using layers of different technologies Further treatment of these services isprovided in Section 3.3

2.1.2 A Simple Technology Primer

The basic Internet transport service carries information packets between end-points of theInternet in much the same way as the post office delivers letters Letters that are going tothe same city are sorted into large mail bags, which are loaded onto airplanes, and thendelivered to a central point in the destination city The letters are then regrouped into thesmaller mail bags that postmen can carry on their routes

Just as the post office uses airplanes, vans and foot, and different size containers and mailbags, so Internet transport service uses many different transport technologies These includeEthernet, Asynchronous Transfer Mode (ATM), Synchronous Digital Hierarchy (SDH),Synchronous Optical Network (SONET), and Dense Wavelength Division Multiplexing(DWDM) These technologies are described in Sections 3.3.2–3.3.5 We introduce the basictechnologies in an informal way that motivates their particular use

For the moment, we emphasize the fact that each of the above technologies provides

a well-defined transport service and packages information in different size packets Thepackets of one service may act as containers for packets of another service Suppose, forsimplicity, that the post office transports fixed size packets between customers A transportcompany provides a container service between local post offices at A and B by runningsmall vans of fixed capacity at regular intervals between A and B Prior to the departure

of a van from A, the local post office fills the van with the packets that are waiting to be

delivered to the post office at B Such a service is a paradigm of a synchronous container service, since it operates at regular intervals and hence offers a fixed transport capability

between point A and B The SONET or SDH services are examples of synchronous services

in communications networks

If each van can hold at most k packets then the unit of information transfer between points A and B is a container of size k If a van departs every t seconds, then the capacity

of the container service is k =t packets per second (For data, we measure capacity in bits

per second, or kilobits, Megabits or Gigabits per second.) Observe that containers may not

be filled completely, in which case the extra space is wasted We can extend this type ofsynchronous container service by supposing that the transport company uses larger vans, of

container size 10k, again leaving every t seconds These containers can be filled by smaller

‘subcontainers’ of sizes that are multiples of k, and customers can rent such space in them (provided that the sum of the sizes of the subcontainers does not exceed 10k) The post office could obtain the same service as before by renting a subcontainer service of size k.

Similarly, an operator running a 622 Mbps SONET service between points A and B can

26 NETWORK SERVICES AND CONTRACTSsell four distinct 155 Mbps SONET connections between these points (after reserving two

of the 622 Mbps to control the connection)

What happens if customers cannot effectively fill the smallest size subcontainers? Saythe post office traffic between points A and B has a maximum rate of 0:5k=t packets persecond, and so can justify using containers of size at most 0:5k, but there are other potential

customers who can also use fractions of k Then there is a business opportunity for another operator, who buys the k container size service from the original operator and reserves

space in each such container for his customers This is a ‘value-added’ service, in the sensethat he may reserve a different maximum amount of space for each customer, fill the unusedspace of one customer with excess traffic of another customer, and is able to distinguishpackets belonging to different customers when the container is unpacked The equivalent

of this ‘smart container packing’ service is an ATM virtual path service A simple case of

container packing is to reserve a fixed portion of the space to each customer For instance,

an ATM service provider using the 155 Mbps SONET service between points A and B,can provide two independent ATM virtual path connections of sizes 55 and 100 Mbps thatmay be sold to different customers Basically, he can flexibly construct any number of suchfixed bandwidth bit pipes based on the actual demand Again notice that a customer such

as the post office which buys the above fixed bandwidth service may not fill the capacity ofthe service at all times There are more interesting ways that ATM can pack the containers

to avoid unused space In these cases, the virtual paths do not have a fixed static size butcan dynamically inflate or deflate according to the actual number of packets that are beingshipped

In the above, the post office plays an analogous role to IP Since the local post office at

B may not be the final destination of a packet, but only an intermediary, the post officer

at B must look at each packet in turn and decide whether to deliver it locally or forward

it to another post office location This is the functionality of the IP protocol: to distinguishpackets belonging to different customers and deliver them or route them effectively throughthe other ‘IP post offices’ A customer delivering packets at random irregular intervals tothe IP post office (destined for some other customers) views the IP service as building aflexible ‘packet pipe’ through the network that does not reserve some predetermined amount

of bandwidth Note that such connections may have highly variable durations, and theirend-points may be unpredictable as far as the IP service is concerned

In its turn, the IP service provider can sell a number of such packet connections betweenpoints A and B (or the capability for activating such connections), by making certain thatthere is only a small probability of completely filling the fixed bandwidth service that hepurchases from the ATM service provider between A and B Now statistics come into play.Since most of the time only a small number of the IP connections will be sending packets

simultaneously, say a fraction p of the total number n, he needs only enough bandwidth between A and B to accommodate pn sources, assuming that these send continuously Note

the large saving in bandwidth compared to what he would need if he were to reserve the

maximum bandwidth needed by each source, that is, enough bandwidth for n such sources instead for pn This controlled overbooking is an effect of statistical multiplexing discussed

in Section 4.2 It is important to observe that fixed bandwidth services can be used for

achieving the reverse effect of flow isolation For instance, if the IP service needs to assign

dedicated bandwidth for a packet connection between A and B, then rather than mixingthese packets with IP packets from other connections in the same containers, it can purchase

a dedicated container service, solely for carrying the packets it wishes to isolate Such flowisolation may be used to guarantee good performance, since shared containers have fixed

A CLASSIFICATION OF NETWORK SERVICES 27

ATM virtual path

is temporarily unused by other connections IP is used to establish connections between arbitrary

network end-points, of unpredicted duration and intensity

size and packets may have to queue at the IP stations to find free space in containers Thiscongestion effect is reduced by offering such an exclusive treatment, but comes at an extracost We are ready now to proceed with the Internet analogy

In the late 1990s, many parts of the Internet were implemented as IP over ATM ATM canrun over SDH (or SONET), which in turn can run over an optical network This transportservice layering is shown in Figure 2.1

More specifically, an optical network technology provides a point-to-point synchronous

‘container’ service, such as SONET operating at a maximum steady rate of 10 Gbps In turn,SONET provides subcontainer transport services with rates that are multiples of 155 Mbps.ATM is used to provide flexible partitioning of such large SONET containers for servicesthat require fractions of this bandwidth IP is responsible for packing and unpacking thefixed size bandwidth services provided by ATM into information streams consisting ofvariable size objects (the IP packets produced by user applications), whose resulting bitrates are much smaller and bursty IP is a multiplexing technology that ‘buys’ such fixedsize bandwidth services and makes a business of efficiently filling them with informationstreams that are variable in both the rate and size of packets Thus, IP and ATM can beviewed as ‘retailers’ of ‘wholesale’ services such as SONET

Different parts of the overall network may be connected with different containertechnologies The idea is to choose a technology for each link whose container sizeminimizes wasted space in partially packed containers In the interior of the network manytraffic streams follow common routes and so it makes sense to use large containers forlinks on these routes However, at the periphery of the network it makes sense to use smallcontainers to transport traffic from individual sources Thus the business of a networkoperator is to provide connectivity services by choosing appropriately sized containers forthe routes in his network, and then to efficiently pack and unpack the containers TheInternet transport service efficiently fills the large fixed size containers of the lower-levelservices and connects two end-points by providing a type of connecting ‘glue’

Example 2.1 (IP over ATM over SONET) A concrete example of transport servicelayering is shown in Figure 2.2 In this figure Provider 1 aims to fill completely his

622 Mbps container service between points K and L He may be buying a light path

service from a provider who owns the fibre infrastructure between the above points, inwhich the container service could run up to 10 Gbps He fills his containers by selling

28 NETWORK SERVICES AND CONTRACTS

Provider 1: network K−L, Provider 2: network G−H, Provider 3: network E−F

Figure 2.2 An example of transport service layering Transport service Provider 1 operates a

622 Mbps SONET service between points K and L and sells 155 Mbps SONET services to customers Provider 2 runs an ATM over SONET network with nodes G, H , and sells a 100 Mbps ATM service between points E and F to Provider 3; to do this he buys a 155 Mbps SONET service for connecting G and H from Provider 1 Provider 3 sells IP connectivity service to customers A,

B, C and D by connecting his routers E and F using the 100 Mbps ATM service

bought from Provider 2

smaller container services, in sizes that are multiples of 155 Mbps, such as that which

connects nodes G and H of Provider 2 Provider 3 sells Internet services to his customers and runs a two node IP network between routers at E and F In doing this, he must connect

these nodes so that they can exchange Internet data This data is packaged in variable size

IP packets and is sporadic, with a total rate not exceeding 100 Mbps To connect E to F

he buys a 100 Mbps ATM Virtual Path (i.e a 100 Mbps bit pipe) from Provider 2 Provider

2 uses the ATM technology to subdivide the 155 Mbps SONET container service between

G and H , and so sell finer granularity bandwidth services For instance, he fills the rest of the 155 Mbps containers traversing the F to G link by selling a 55 Mbps ATM connection

to some other customer Note that if Provider 3 has enough Internet traffic to fill 155 Mbps

containers, he can buy a pure SONET service between points E and F, if available This

is what happens in IP over SONET If he has even more traffic, then he can buy a lightpath service to connect the same points, which is IP over ½ Such a service may providefor 10 Gbps of transport capability for IP packets

Note that bitrate is not the only differentiating factor among transport services The

IP network E–G allows any pair of customers amongst A; B; C and D to connect for

arbitrarily short times and exchange data without the network having to configure any suchconnections in advance By contrast, SONET (and ATM) are used for specific point-to-pointconnections that have a much longer lives

Finally, each service that is sold to a customer has initial and final parts that give access

to the provider’s network For instance, in order to run the ATM service between E and F one must connect E to G and H to F This access service may be provided by Provider 2 himself or bought from some third provider Similarly, IP customer A must use some access

service to connect to the IP network of Provider 3

2.1.3 Value-added Services and Bundling

Some services provide much more than simply a data transport service Consider a webservice It provides a data transport service, but also a data processing service and a datapresentation service The latter two services add value and belong to a layer above that

A CLASSIFICATION OF NETWORK SERVICES 29

of the transport service Thus, the web browsing is what we call a value-added service,

which is complementary to the network transport service Similarly, an Internet telephony

service is a bundle of services, which includes a directory service, a signalling service, a

data transport service and a billing service In Section 3.6, we discuss a possible model forInternet services and explain the structure of the value chain in Internet service provisioning

It is important to distinguish between transport and value-added services Think of abookstore which provides the value-added service of retailing books by mail order Acustomer chooses his books and says whether he wishes delivery to be overnight, in twobusiness days, or by ordinary post He pays for the books and their delivery as a bundle, andthe bookstore contracts with a delivery service for the delivery The bundled service hascomponents of attractiveness and timeliness of book offerings, speed of delivery and price.The demand for books drives the demand for the delivery service Similarly, the demandfor information services drives the demand for data transport services How a customervalues the particular content or functionality of a communications service determines thecharge he is prepared to pay Of course, this charge will contain a component that reflectsthe value of the data transport service, since transport service is what a communicationsnetwork provides In Figure 2.3 the user enjoys a video on demand value-added service.Although the customer may make a single payment for the service (to download the softwarerequired, run the application and watch the movie at a given quality level), this payment may

be further split by the valued-added service provider to compensate the transport serviceprovider for his part of the service

It is useful to familiarize oneself with some of the formal definitions that regulators use toclassify network services The Federal Communications Commission (FCC) uses the term

information services for value-added services, and telecommunications services for level transport services The Telecommunications Act of 1996 defines telecommunications

lower-as “the transmission, between or among points specified by the user, of information ofthe user’s choosing, without change in the form or content of the information as sent and

received”, and a telecommunications service as “the offering of telecommunications for a

fee directly to the public, or to such classes of users as to be effectively available to the

public, regardless of facilities used” An information service is defined as “the offering of a

capability for generating, acquiring, storing, transforming, processing, retrieving, utilizing,

or making available information via telecommunications” According to these definitions, an

transport service interface

add value to transport service

video player

network IP interface

communications socket

exchange application data video server

Figure 2.3 Transport and value-added services The user enjoys a value-added service (such aswatching a movie) which combines the transport of data from the video server with the contentitself, and probably some additional functionality from the video server (such as back-track,fast-forward and pause) Such an application may require some minimum bitrate in order to

operate effectively

30 NETWORK SERVICES AND CONTRACTSentity provides telecommunications only when it both provides a transparent transmissionpath and it does not manipulate the form or content of the information If this offering ismade directly to the public for a fee, it is called a ‘telecommunications service’ An entitymay sell an information service as a bundle of telecommunications (the lower-level datatransport services) with content specific applications such as email and web browsing (thevalued-added services according to our previous definitions), or sell telecommunicationsseparately as independent services According to this definition, telecommunications refers

to the lower end of the network transport services, where the network offers transparentbit pipes When, as with TCP/IP, data is processed either inside the network at the routers,

or at its edges, the resulting service is closer to an information service according to thisdefinition In practice, information services are more usually viewed as being associatedwith content and value-added applications that run at the edges of the network In theInternet, such applications manipulate the data part of the IP packets according to theparticular application logic Network transport, such as the routing of IP packets, is notconsidered a valued-added service, as it is offered as a commodity, using open standards

In this book, we deal with network transport services that complement these higher-level,value-added applications By the FCC definitions they are ‘telecommunications services’ atlower layers and ‘information services’ at higher layers

2.1.4 Connection-oriented and Connectionless Services

We may also classify services by the way data is transmitted In a connection-oriented service, data flows between two nodes of the network along a ‘virtual’ pipe (or a tree of

virtual pipes when multicasting, with duplication of data at the branching points) Data

travels along a fixed route, with a specified rate, delay and error rate In a connectionless service, the data does not follow a fixed routing Instead, the data is transmitted in packets,

or datagrams Successive datagrams, travelling between a source and destination, can takedifferent routes through the network, and can suffer loss

Connection-oriented and connectionless services may not be substitutable It is moredifficult, or impossible, for a connectionless service to deliver datagrams in a regular way.Take, for example, the postal service, which is a datagram service It ensures that parcelscan be sent to a destination from time to time, with acceptable delay However, it cannotguarantee delivery of a stream of parcels to a destination at a constant rate, say one perhour That would require a connection-oriented approach in which a flow of packets istreated as a separate entity A schematic of connection-oriented and connectionless services

is shown in Figure 2.4

A connection-oriented service can be used to provide a type of deterministic performanceguarantee Consider a connection and a link of the network that it uses Suppose we reserveperiodically reoccurring slots of time on this link for transmission of the connection’spackets It is as if the link were a conveyor belt, and a fixed portion of the belt werereserved for carrying the connection’s packets Each time that portion comes around one

of the connection’s packets can be sent We assume that slots are large enough to carry anintegral number of packets In practice, packets may be fragmented into small fixed size

pieces (called cells), where a slot of the synchronous link (the belt) is large enough to hold

a cell Slots reoccur, being part of larger constructs called frames For instance, a particular

connection might be assigned the first two slots in a frame consisting of hundred slots, suchthat every hundred slots the connection gets the first two slots Packets are reconstructed

at the end from the corresponding cells If the connection sends a stream of packets at a

A CLASSIFICATION OF NETWORK SERVICES 31

service connects A to B and C with a bit pipe of R Mbps, maximum delay T and bit error rate r

In (b) a connectionless service delivers a message of size M to B and C with maximum delay T

and loss probability p.

constant rate, and sufficient time slots are reserved on all the links that it uses, then itspackets will arrive at the destination at that same constant rate Such a transport service is

called a synchronous service An important characteristic of synchronous service is that the

relative timing of packets at the entrance is preserved at the exit

This is in contrast to an asynchronous service, which makes no such static allocation

of slots to connections Slots are allocated on demand, only when cells are present to becarried A key characteristic of asynchronous service is that the relative timing of packets

at entrance and exit is not preserved However, we can have reservation of resources (atpossibly less than maximum rate) even for asynchronous services (e.g ABR with MCR,VBR, etc.) For instance, we may specify that every 100 slots, the connection should beable to get at least one slot (if it has cells waiting to be transferred) Note that no particularslot is reserved solely for use of the connection

Continuing the conveyor belt analogy, when a packet is to be placed on a belt theremust be an empty slot, but specific slots on the belt are not pre-allocated to connections Ifthere is contention for slots, then the connections must wait for the network to assignthem free slots The network does this using some ‘contention resolution policy’ We

define synchronous networks as those that support only synchronous services They use

technology that is optimized for this purpose, breaking information into packets of thesize that can be transmitted in a slot and then sending them as streams of slots whilereserving specific slots for each connection on the links that the connection uses In

contrast, packet switching (or cell switching) technology is used for asynchronous services.

Information is broken into variable or fixed-sized packets, called cells These are transported

in a store-and-forward manner, without preallocating any slots Examples of synchronousnetworks are ISDN (Integrated Services Digital Network), SDH and SONET Examples ofasynchronous networks are the Internet, Frame Relay and ATM Note that a synchronousservice can be provided by an asynchronous network (such as a CBR service in ATM)

by performing smart scheduling of the slots This is used for running telephony overATM

Clearly, if customers send data sporadically then a synchronous service may be inefficient,since preallocated slots can go unused Asynchronous services are better Note that because

of the sporadic nature of asynchronous services it may be sensible for the network to engage

in some sort of ‘overbooking’ when assigning resources to slots For instance, one mayassign a number of slots that is less than what would be required to support the peak rate

of the connection

32 NETWORK SERVICES AND CONTRACTS

2.1.5 Guaranteed and Best-effort Services

There is an important distinction between services that do and do not come with guarantees,and which correspondingly do and do not require some reservation of resources On the one

hand, guaranteed services come with quality of services guarantees that are expressed in

terms of certain parameters of the service’s performance Some reservation of resources isusually required if the guarantees are to be fulfilled For example, a service that guarantees

a minimum transmission rate may need to reserve capacity on a set of links On theother hand, a service may make no guarantees and reserve no resources; in this case, theperformance of the service depends on the quantity of resources it is allocated, and thisallocation depends on the network’s policy and the set of other services that compete forresources Since the network usually tries to provide the best quality it can to each of its

customers, these services are called best-effort services.

Service guarantees may allow some flexibility For example, it might be guaranteed that

no data will be lost if the user’s sending rate never exceeds h, but subject to the network being allowed to vary the posted value of h For more details see Section 2.2.1 This

type of flexibility can help the network to improve efficiency by making better use ofresources

The request for a network service originates at an application, and so it is the application’sneeds that determine the type of connection required to exchange information For example,

a video server needs a minimum bandwidth to send real-time video and so needs aguaranteed service Other audio and video applications can tolerate performance degradationand can adapt their encoding and frame rates to the available bandwidth They are examples

of elastic applications For these, flexible guarantees may be acceptable.

Note that an elastic application must know the bandwidth that is available at any giventime and be able to adapt its rate, rather than risk sending information into the networkthat may be lost Thus application elasticity goes hand-in-hand with the network’s ability

to signal resource availability Elastic services require this signalling ability Best-effortservices usually do not provide signalling and so elastic applications must implement thissignalling functionality themselves (at the application layer) Thus guaranteed services,which provide flexible guarantees, such as in the example above, may be better suited tosome adaptive applications

Example 2.2 (Traditional Internet transport services) The Internet Protocol (IP) is the

basic protocol by which packet transport services are provided in the Internet It operates

as a simple packet delivery service When the IP ‘representative’ (a piece of software) atthe source machine is handed a packet of data and the address of a destination machine (an

IP address), it forwards this packet tagged with the IP destination address to ‘colleagues’(IP software) running on Internet computers (the routers) These continue to forward androute the packet until it reaches the IP representative at the destination machine If thenetwork is congested, then packets may be lost before reaching their destination Thishappens when a packet arrives at a router and overflows the available storage This classic

IP service is a best-effort service, because it provides no performance guarantees Today’srouter implementations permit certain IP packets to receive priority service However, noexplicit guarantees are provided to the flows of such packets

TCP (Transport Control Protocol) and UDP (User Datagram Protocol) are two transportservices that run on top of the IP service, and so are denoted by TCP/IP and UDP/IP TheTCP/IP protocol provides a data transport service with certain performance guarantees Itguarantees zero packet loss to the user by retransmitting packets that are lost because of

SERVICE CONTRACTS FOR TRANSPORT SERVICES 33congestion inside the network The basic idea is that the TCP software ‘listens’ to congestionindication signals transmitted by the network and intelligently adjusts its sending rate to theminimum capacity available in the links along the path Any lost packets are resent Theprotocol aims to minimize such retransmissions and to achieve a high link utilization, but itguarantees no minimum or average sending rate Essentially, the rate at which a connection

is allowed to send is dictated by the network TCP/IP has the interesting property thatwhen it is used by all competing connections then the bandwidth of the bottleneck links

is fairly shared (In practice, connections with longer round-trip delays actually receivesmaller bandwidth shares since they are slower to grab any extra bandwidth This can havesevere repercussions)

A connection using the UDP/IP protocols has no constraints, but also has no guarantees.UDP adds little functionality to IP Like TCP it allows the receiver to detect transmissionerrors in the data part of the packet It sends at a maximum rate, irrespective of congestionconditions, and does not resend lost data UDP is appropriate when one wants to send asmall burst of data, but because of its short life, it is not worthwhile to set up a completeTCP/IP connection UDP is a typical example of a best-effort service with no guarantees.Further details of these protocols are provided in Section 3.3.7

2.2 Service contracts for transport services

A transport service is provided within the context of a service contract between network

and user A part of the contract is the tariff that determines the charge Beyond this, the

contractual commitments of the network and user are as follows The network commits to deliver a service with given quality and performance characteristics, and the user commits

to interact with the network in a given way.

If the user violates his side of the contract, then the contract might specify what thenetwork should do The network might not be bound to any quality of service commitment,

or it might restrict the service quality given to the user Service quality characteristicsinclude geographic coverage, billing services, reliability, up-time, response to failures, help-desk and call-centre support Service providers can differentiate their service offerings bythese characteristics, and so influence the customers’ choices of provider Not surprisingly,

it is often hard to quantify the costs of providing these characteristics No standards exist

to constrain the definition of a service contract It may be arbitrarily complicated and mayinclude clauses specific to the customer

The part of the service contract that deals with the valued-added part of the service can

be complicated, since it can concern issues that are specific to the particular value-addedapplication, such as the copyright of the content provided Throughout this book, we mostlychoose to focus on that part of the service contract that concerns the quality of the transport

service We call this the traffic contract part of the service contract For simplicity, we speak

of ‘service’, rather than ‘transport service’, when the context allows

2.2.1 The Structure of a Service Contract

Let us focus on the traffic contract part of the service contract, i.e the part that dealswith aspects of the transport service In practice, this part of the contract can be describedindependently of the particular network technology

As a first example, suppose the network agrees to transport cells between two given

points, at a rate no less than m, and dropping no more than a proportion of cells, p The user agrees to send cells at no greater than a rate, say h, and to access the network for

34 NETWORK SERVICES AND CONTRACTS

no longer than a maximum time f If the user sends a total of V bits he will be charged

a C bV If the user exceeds his contract limits, the network will provide the excess of the

user’s traffic with a free and simple best-effort service, but this has no guarantees The

parameters of the traffic contract are m, p, h, f , a and b.

Now let us be more formal Both the user’s traffic and the service quality provided by

the network have properties that can be quantified in terms of measurable variables They

include, for example, the present rate at which data is sent, the average rate so far, therate at which cells are lost, and the average delay per cell (In practice, many contractvariables are not measured, as it is too costly to provide the necessary infrastructure This

‘information asymmetry’ can provide incentives for contract violation, see Chapter 12.)

Let P and Q be predicates over the variables, where P denotes a constraint on the user’s traffic and Q denotes a constraint on the performance provided by the network Then the service contract can be represented as P H) Q The parameters of the contract are constants used in the construction of P and Q For instance, the contract clause ‘if the

sending rate is less than 2 Mbps, then maximum delay per packet is guaranteed to be less

than 10 ms’ can be specified as ‘x1  2 H) x2  10’, where x1, x2 are variablesand 2, 10 are constants Conceptually, one can imagine that the current values of thevariables and constants are continuously displayed at the service interface between userand network

A service contract is sometimes called a Service Level Agreement (SLA) This

terminology is more often used for traffic contracts between large customers and networkoperators, and includes parameters specifying help desk and customer support In practice,SLAs are specified by long textual descriptions and may contain many ambiguities

The network’s contractual obligations can take the form of either deterministic or statistical guarantees An example of a deterministic guarantee is a strict upper bound on

delay Another example is 100% service availability Statistical guarantees can be framed

in terms of cell loss rate, the probability of obtaining access to a modem bank, or theprobability that a web server will refuse a connection due to overload The idea that thenetwork should make some service level guarantees represents a departure from the basicbest-effort model of the Internet In Example 2.3 we describe such guarantees in the context

of a particular technology

Example 2.3 (Quality of Service) The quality of service (QoS) provided by a

transport service is defined in terms of the way a traffic stream is affected when

it is transported through the network This is typically in terms of the probability

of cell loss, delay, and cell delay variation (or jitter) If access to a resource isrequired, it may also include the probability that service is refused because theresource is not available In the case of ATM services (see Section 3.3.5), the QoSmeasures are

Cell Loss Ratio (CLR): the proportion of cells lost by the network.

Cell Delay Variation (CDV): the maximum difference in the delays experienced by two

different cells in their end-to-end transit of the network CDV is also known as

‘jitter’

Maximum Cell Transfer Delay (max CTD): the maximum end-to-end cell delay.

Mean Cell Transfer Delay (mean CTD): the average end-to-end cell delay.

Minimum Cell Rate (MCR): the minimum rate at which the network transports cells.

SERVICE CONTRACTS FOR TRANSPORT SERVICES 35

A popular way to specify a user’s contractual obligation concerning the traffic he sends

into a network is by a leaky bucket constraint This is described in Example 2.4 The

constraint is on the source’s peak rate, average rate and burstiness

Example 2.4 (Leaky buckets) The leaky bucket traffic descriptor can be used to bound

the density of a traffic stream at a reference point in the network It constrains the trafficstream’s peak rate, average rate and burstiness (i.e the short range deviations from themean rate) Suppose the unit of traffic is a cell A leaky bucket descriptor is defined by a

leak rate, r , and bucket size, b Let X [t ; t0] denote the number of cells of the traffic stream

which pass the reference point during the interval [t ; t0/ The leaky bucket imposes the

constraint that a conforming traffic stream must satisfy

Note that as the window [t ; t0/ increases in width, the average rate permitted during this

window becomes bounded above by the leak rate r , but that for a window width of t0t the stream is allowed to produce a ‘burst’ of size b above its greatest allowed average

amount of .t0t /r Since this amount b can be produced within an arbitrarily small time

window, the leaky bucket descriptor does permit an arbitrarily large peak rate Note that

large values of b allow for large bursts However, b D 0 places a simple bound on the peak rate; at no point can it exceed r

The simplest way to police a source is by the speed of the access line to the network

This simple mechanism is equivalent to a leaky bucket with b D 0 and r equal to the line

rate Figure 2.5 depicts a leaky bucket

More complicated contractual obligations can be specified from logical combinations

of simpler ones In Example 2.5 the contractual obligation is specified as the conjunction

of two simple leaky bucket constraints, each of which addresses a different aspect of the

traffic Such constraints upon the user’s traffic are also called traffic descriptors.

Example 2.5 (Multiple leaky bucket traffic descriptor) For traffic that is bursty (i.e.

which has phases of high and low activity), it is customary to use two leaky buckets tospecify conforming traffic The first leaky bucket constrains the peak rate and the secondleaky bucket constrains the time for which the source can send a burst at the peak rate An

Figure 2.5 A leaky bucket policer A real cell is conforming if and only if when it arrives there is

space in the token buffer to add a virtual cell The token buffer has space for b virtual cells and is

depleted at constant rate of r cells/s.

36 NETWORK SERVICES AND CONTRACTS

example of this is the VBR traffic descriptor shown in Figure 2.6 It is used to characterizebursty ATM traffic (described in Chapter 3)

An arriving cell is conforming when it is conforming to both leaky buckets The leakybucket that constrains the peak rate is defined in terms of the PCR (peak cell rate) and CDVT(cell delay variation tolerance) When CDVT D 0, then the minimum time that is allowedbetween two cell arrivals is 1=PCR, whereas if CDVTðPCR D 0:1 (a typical case), then thisinterarrival time may be temporarily 0:9=PCR This allows some small fluctuation in cell in-terarrival times, but the average rate at which cells arrive cannot exceed PCR Similarly, theleaky bucket that constrains burstiness is defined in terms of SCR (sustainable cell rate) and

BT (burst tolerance) Usually the value of BTðSCR is a large integer, allowing for a burst ofcells to arrive at greater rate than SCR Note that the allowed duration of the burst increaseswith BT and depends on the rate at which cells arrive Since the average rate of conformingcells cannot exceed SCR, this leaky bucket also constrains the mean rate of the source.The Integrated Services architecture for the Internet uses a similar approach in its trafficspecification (Tspec) This is defined in terms of a dual leaky bucket (similar to the aboveand with CDVT D 0), a bound on maximum packet size (since in the Internet packet sizes

are not fixed as they are in ATM), and a ‘minimum policed unit’ m (specifying that packets smaller than m bytes should be padded to size m when entering the leaky bucket) There

is also a similar use of leaky buckets in the Differentiated Services architecture for theInternet For more details of these architectures, see Section 3.3.7

2.2.2 Policing Service Contracts

The network must take steps to monitor and enforce the user’s conformance to his

contractually agreed interactions with the network This is called policing the contract.

For example, a telephone network might wish to police its users for an agreed maximumfrequency of dialling, so as to prevent an overload of the signalling part of the network

It is necessary to say what will happen if the user violates his part of the contract Thiscan be specified in the service contract itself One possibility is to specify that if there is aviolation then a different quality of service will be provided For example, if conformance

of the user’s traffic stream is being policed by a leaky bucket, then a simple specificationwould be that if there is congestion in the network then it can discard any non-conformingcells, either as they enter or traverse the network The network could mark non-conformingcells, so that they can be the first to be dropped when congestion occurs

Of course, there is a cost to policing and it is desirable that policing be implementedsuch that it introduces low overhead and on the basis of measurements that are easy tomake The user has the incentive to produce traffic that conforms to the contract One way

SERVICE CONTRACTS FOR TRANSPORT SERVICES 37

to do this is by policing his own traffic prior to delivering it to the network When cellsare produced that do not conform to the contract, these are placed in a buffer until they

become conforming This traffic shaping tends to smooth the traffic.

A simple way to provide a loose form of policing is by charging The network simplyprovides an incentive for the user to respect the contract by imposing a very high chargewhenever he violates it This method has the advantage that it acts indirectly, rather than

on a cell by cell basis Of course it can only work if charges are based upon measurements

of the user’s traffic that capture the contract violations Although costly to implement, theflexibility allowed by this type of policing may be of great value to those applications thatwould sometimes prefer to pay a bit more, rather than see their traffic trimmed by the leakybuckets Multimedia applications may fall in this category High capacity networks, servinglarge numbers of contracts, gain from the fact that traffic streams do not always fully utilizetheir contracts It is possible that the provision of a more flexible service contract does notchange the load of the network significantly, but does greatly increase the value of theservice to the customers

2.2.3 Static and Dynamic Contract Parameters

There are many service quality characteristics for which no explicit guarantees are made inthe traffic contract The network is free to address them in a best-effort way, or according

to some other internal policy For example, an Internet service provider might make noguarantee as to the probability that a user will find a free modem when he dials-in, butoperate enough modems such that he receives very few complaints

When service contracts do make explicit guarantees, they can do so in different ways.These can differ in the parameters in which they are expressed and the commitments thatthey require from the network Consider the following three contracts:

There will be no data loss

provided the rate of the

source stays below

1 Mbps

There will be no data lossprovided the rate of thesource stays below

h Mbps, where the network can vary h

dynamically between 1and 2 Mbps

The data loss rate will

be less than0.000001% providedthe rate of the source

stays below h Mbps,

where the user can

vary h dynamically

between 1 and 2 Mbps

Contract A expresses a guarantee in terms of static parameters, i.e ones that are set at

the time the contract is established and remain constant throughout its life This guaranteerequires the network to make a firm commitment of resources The network must reserve

1 Mbps for the service at the start of the contract Contracts B and C express their guarantee

in terms of both static and dynamic parameters Dynamic parameters are ones that are

updated during a contract’s life Contract B has a static part, guaranteeing 1 Mbps Contract

C has a static part defined in terms of static parameters 1 Mbps and 0.000001% Both have

an extra, purely dynamic part, for extra rate between 0 and 1 Mbps A significant difference

between B and C is that C is lossy even with rate below h Mb/s.

Apart from the loss guarantee in Contract C being statistical, the main difference between

Contracts B and C lies in who chooses h: